Chant was THE music of worship for Jews and Christians until after 1500; it remains the "normative" music of the Catholic and Orthodox churches, is sometimes used by Anglicans and Presbyterians, and is a source for many modern hymn tunes.
Unlike hymns (and almost all other music heard in the modern Western world), Chant rhythm is not based on regular meters traceable back to dance forms, but on the rhythm of the words being sung. Chant also retains a notion of harmony fundamentally different from the keys, modulations, and even scales typical of Western art and popular music.
[Editor's note: this category primarily includes links to collections of links, as I know very little of the subject. Fortunately, there are several excellent link collections out there. Also, I'd encourage anyone with interest and knowledge of the subject to donate their time to this part of the Open Directory Project.]